The present invention relates to electric machines, and more particularly to spoke permanent magnet rotors for use in electric motors.
Spoke permanent magnet rotors generally include a plurality of permanent magnets radially arranged around a shaft. The permanent magnets are magnetized tangentially, through the thickness of the permanent magnet, either before or after insertion into the rotor. Magnetic flux generated by the permanent magnets is collected and concentrated by ferro-magnetic pole pieces. Often, the pole pieces are utilized to retain the permanent magnets.
Some examples of spoke permanent magnet rotors are disclosed in U.S. Pat. Nos. 4,631,807; 5,140,211; 5,157,297; 5,200,662; and 5,463,262.
Spoke permanent magnet rotors have a natural flux concentration capability because two circumferentially adjacent permanent magnets contribute to the air-gap flux for each rotor pole. This flux concentration capability allows the air-gap magnetic flux density to be higher than the flux density of each permanent magnet that contributes to the air-gap magnetic flux density. Accordingly, electric motors that include spoke permanent magnet rotors often have a high specific torque output. This translates to a smaller overall weight and lower material cost for a given rating of electric motor. Further, based on the radial arrangement of the permanent magnets, the permanent magnets are less likely to become demagnetized. Therefore, rated operation close to the maximum energy product is possible without the risk of demagnetization of the permanent magnets under fault conditions. Operation at a maximum energy product ensures optimum utilization of the permanent magnets. This translates to lower material costs for a given rating of electric motor.
Despite the above-mentioned advantages of spoke permanent magnet rotors, only a relatively small number of spoke permanent magnet rotors are produced worldwide. One limiting factor for high volume production is the complicated construction required to reduce or eliminate the permanent magnet flux leakage from the radially inward portions of the permanent magnets towards the magnetic shaft. Another limiting factor is the complicated construction required to retain the permanent magnets and the pole pieces in the rotor during normal operation of the electric motor. A new spoke type permanent magnet rotor that provides enhanced performance and reduced costs would be welcomed by those in the art.